Non-linear radar range scale display arrangement

ABSTRACT

A non-linear range scale for a radar system is implemented whereby the full range capability of the system is shown with emphasis on close-in targets. The arrangement is adaptable to a variety of non-linear functions having the advantage of selection by the user to provide good close-in visibility and visibility of distant conditions simultaneously.

BACKGROUND OF THE INVENTION

Users of radar systems experience a problem when, for example, using theradar system for guiding an aircraft through thunderstorms and otheradverse weather conditions. Such users are aware of adverse weatherconditions in close proximity to the craft. In order to be aware ofadverse weather conditions beyond a presently selected range scale ofthe radar system, the user needs to constantly switch from close-in todistant range scales. Failure to do this for one reason or another canresult in potentially hazardous flying situations.

Prior art radar systems utilize linear range scales. That is to say, thedisplayed distance is directly proportional to the actual distance. Thepresent invention, on the other hand, features a non-linear range scalewhich emphasizes the close-in range relative to a more distant range.With this arrangement, close-in targets such as weather disturbances aredisplayed larger and with substantial detail while more distantdisturbances are displayed in a manner so as to alert the user to same.

SUMMARY OF THE INVENTION

This invention contemplates a non-linear radar range scale displayarrangement wherein close-in targets are displayed with substantialdetail, while more distant targets are simultaneously discernable. Forexample, when using a radar system for detecting weather disturbances, adisplayed weather pattern might be that of a long frontal weatherdisturbance. With prior art radar systems, only the first ten miles orso in front of the system antenna/receiver would be seen, perhapsshowing only a single target or weather cell. To alleviate thissituation, the scale displayed in accordance with the present inventionis proportional to the logarithm (log) to the base ten of the range (R).The displayed scale can be commensurate with a variety of non-linearfunctions in addition to log R as aforenoted, such as the square root ofR, or R^(a), where "a" is a positive value less than one. The selectionof a specific non-linear function is dependent upon how much emphasis isrequired on close-in versus distant ranges.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic plan view representation illustrating aparticular non-linear radar range scale.

FIG. 2 is a graphical representation illustrating displayed distance (D)versus radar range (R) for the non-linear functions log R, R⁰.5 and R⁰.1as contemplated by the invention.

FIG. 3 is a block diagram of a radar system incorporating the featuresof the invention.

FIG. 4 is a plot of the piece-wise linear approximation of range binsversus range for the particular non-linear function log R.

FIG. 5 is a block diagram particularly showing a log clock generatorshown generally in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

In implementing the invention, values for a plurality of range rings aredetermined for a non-linear scale. With reference to FIG. 1, four suchrings are shown and are designated by the numerals 2, 4, 6 and 8. Rangering 8 is at full distance, range ring 6 is at three-quarter distance,range ring 4 is at one-half distance and range ring 2 is at one-quarterdistance. Part of a target such as a weather cell 5 is within ring 2;the rest of target 5 and most of a target 7 are within ring 4; the restof target 7 and targets 9, 11, 13 and part of a target 15 are withinring 6; and the rest of target 15 and targets 17 and 19 are within ring8. The weather pattern depicted might be typical of a long frontalweather pattern. With a non-linear range scale the close targets areshown in detail, but it can be discerned that the front extends for along distance in front of an observer. With the above in mind, considerthe following:

    D=(log R)/K,                                               (1)

where D equals displayed distance (1.0 is full scale); R equals radarrange in nautical miles (nm) and K is a constant to normalize the radarrange to the displayed range.

For outer ring 8, D equals 1. Thus substituting appropriate values inequation (1), the following is obtained:

    1=(log 320)/K,                                             (2)

where K equals 2.505.

For ring 6, log R equals 0.75×2.505, or R equals 75.7 nm, nominally 80nm, as shown in the Figure. Similarly, for ring 4, log R equals0.5×2.505 or R equals 17.9, nominally 20 nm, and for ring 2 log R equals0.25×2.505, or R equals 4.2, nominally 5 nm, as also shown in FIG. 1.

In order to judge as to which non-linear function would be mostdesireable for use with a particular radar system, several non-linearfunctions are plotted, as illustrated in FIG. 2. Thus, FIG. 2 showscurves obtained by plotting displayed distance against radar range forthe following three non-linear functions: log R; R ⁰.5 ; and R⁰.1. Forpurposes of illustration, the selected function for implementation isthe log R function, since this function appears to provide the bestcompromise of resolution and display area utilization, as is desireable.

To best understand the aforenoted implementation, reference is made tothe weather radar system block diagram illustrated in FIG. 3. Thus, atransmitter is designated by the numeral 10. Transmitter 10 may be amagnetron which is connected to an antenna/receiver 12 via a circulator14.

Pulses are directed from antenna 12 to a target such as a weather cellas shown in FIG. 1. The pulses are reflected from the target toantenna/receiver 12 and are directed to a preamplifier 16 via circulator14 so as to provide an amplified signal. The amplified signal is mixedby a mixer 18 with a signal from a local oscillator 20. Mixer 18 mixesthe frequencies of the amplified signal from pre-amplifier 16 and thesignal from local oscillator 20 to provide an intermediate frequency(IF) signal. Local oscillator 20 is driven by an automatic frequencycontrol (AFC) signal provided by a microprocessor 21. Microprocessor 21also provides a START signal and a RESET signal for purposes to behereinafter described. Signal AFC is a digital signal which is convertedto an analog signal by a digital to analog (D/A) converter 23 and theanalog signal drives the local oscillator. The mixed signal from mixer18 is amplified and filtered by an amplifier/filter 22. The amplifiedand filtered signal is detected by a detector/discriminator 24.

Detector/discriminator 24 provides an analog signal which is applied toa sample and hold circuit 25. Sample and hold circuit 25 is responsiveto the START signal from microprocessor 21 for applying a sampled andheld signal to the microprocessor.

The analog signal from detector/discriminator 24 is converted to adigital signal by an analog to digital (A/D) converter 26 which isdriven by a log clock generator 30. The digital signal is stored in amemory device 28 to be later applied to a display device 29 in anappropriate format. In this regard, it is noted that display device 28is an external device and is connected to memory device 28 by aself-clocking high speed bus 31.

Log clock generator 30 provides a proper clock frequency to quantize aselected non-linear radar range scale to the actual radar range scale.In this regard, it will be understood that the round trip time for thepulses from and to antenna/receiver 12 is approximately 12.35 μs per nm.To create a log R range scale, a clock generator must be provided thatvaries its frequency logarithmically with time. Log clock generator 30which is responsive to the START and RESET signals from microprocessor21 and which, in turn, drives memory device 28 through A/D converter 26serves this purpose.

The approach taken to configure log clock generator 30 is to use a"piece-wise" linear approximation to the actual clock signal desiredand, in this regard, reference is made to FIG. 4 which is a plot of the"piece-wise" linear approximation of range bins versus range. With eightlinear range segments, the desired log R function is obtained withreasonable accuracy. It is to be noted that this approach is adaptableto any particular non-linear function, with the log R function beingdescribed for illustration purposes.

FIG. 5 is a block diagram of log clock generator 30 shown generally inFIG. 3. Thus, log clock generator 30 includes four basic components: acycles counter 32; encoding logic 34; a divide ratio register 36; and aprogrammable divider 38. The arrangement is such that log clockgenerator 30 provides a pre-set clock frequency for a predeterminednumber of clock cycles and then switches to a frequency at one-half thatrate. The new clock rate is active for a predetermined number of clockcycles before switching occurs to the next frequency, now one-quarter ofthe original. This repeats for a total of eight different frequencies.

The function of cycles counter 32 is to count the number of range binsproduced. The radar system contemplated uses a total number of twohundred fifty-six range bins for a complete range scale. The followingtable outlines the switching points for the overall function of logclock generator 30.

    ______________________________________                                        Segment  Divide Ratio  Range Bins                                                                              Preload                                      ______________________________________                                        1         2            51        255                                          2         4            39        254                                          3         8            38        252                                          4        16            29        248                                          5        32            29        240                                          6        64            39        224                                          7        128           25        192                                          8        256            6        128                                                                 256 total                                              ______________________________________                                    

Encoding logic 34 which is connected to cycles counter 32 via eightcycles counter outputs (LC0-LC7) monitors the output of cycles counter32, looking for a match for the aforementioned switching points andprovides outputs DO-D6. When a switching point is observed, anappropriate preload value is sent to a seven bit divide ratio register36. Divide ratio register 36 holds the preload values listed in thetable above. Divide ratio register 36 drives programmable divider 38through inverters 39A-39G.

A "D" type flip-flop (F/F) 40 is connected at an input D to an overflowoutput (OVF) of cycles counter 32; a "D" type flip-flop (F/F) 42 isconnected at a clear (CLR) input to the output of an OR gate 41; and a"J-K" type flip-flop (F/F) 44 is connected at its J and K inputs to anoverflow output (OVF) of programmable divider 38. OR gate 41 receives anoutput (Q) from flip-flop 40 and the RESET signal from computer 21.

A clock output from a main systems clock 43 which may be a crystaloscillator is applied to a clock input (CLK) of programmable divider 38and is applied through an inverter 45 to a clock input (CLK) offlip-flop 44. All of the 39A-39G inverter outputs are set to a logic "1"by a logic start signal (LSTRT) applied to a reset input of divide ratioregister 36 from flip-flop 42 and are applied to programmable divider38. Signal LSTRT is applied to a clear (CLR) input of flip-flop 44. TheRESET signal from computer 21 clears cycles counter 32 and flip-flop 40and 42, which when applied to a clear (CLR) inputs thereof resets thesystem. A preload value of all logic "1's" causes an overflow condition(OVF) to always be present at the output of programmable divider 38 sothat the output of J - K flip-flop 44 toggles with every clock pulse,creating a divide by two output, i.e. LGCK and LGCK. Output LGCK isapplied to the clock input (CLK) of cycles counter 32 and output LGCK isapplied to the clock input (CLK) of divide ratio register 36 and to theclock (CLK) input of flip-flop 40. As the bits in divide ratio register36 are cleared, the proper preload value is obtained. - With referenceto the chart above, after two hundred fifty-six range bins have beenproduced, cycles counter 32 overflows setting flip-flop 40 which clearsflip-flop 42 through OR gate 41, thus resetting the system awaiting thenext sequence of pulses from microprocessor 21 (FIG. 3) to start anotherlog clock sequence via the START signal from microprocessor 21.

Programmable divider 38 is a binary counter which can be programmed toproduce different output frequencies. The counter starts at a preloadedvalue and then counts up until it overflows. The overflow conditionallows the final stage, flip-flop 44, to toggle, as aforenoted. Theoverflow condition also causes programmable divider 38 to load thestarting value. When this value is left constant, the result is a fiftypercent duty cycle clock signal at a rate of one-half the overflow rateof the counter. Since the value used in the preload is programmable, theoutput frequency is correspondingly programmable. It should be notedthat the arrangement can be used on any range scale by simply changingthe main clock input.

With the above description of the invention in mind, reference is madeto the claims appended hereto for a definition of the scope of theinvention.

What is claimed is:
 1. A non-linear radar range scale displayarrangement, comprising:means for receiving radar signals transmittedfrom a target and for providing signals proportional thereto; oscillatormeans for providing oscillating signals; control means connected to theoscillating means for controlling the frequency of the oscillatingsignals therefrom; mixing means connected to the receiving means and tothe oscillator means for mixing the proportional signals and theoscillating signals and for providing mixed signals; means connected tothe mixing means for detecting the mixed signals and for providingdetected mixed signals; means connected to the detecting means forsampling and holding the detected mixed signals, and connected to thecontrol means and controlled thereby for applying sampled and heldsignals to the control means; clock generator means connected to thecontrol means and controlled thereby for quantizing a selectednon-linear radar range scale to an actual radar range scale and forproviding quantized signals; means connected to the detector means andto the clock generator means and responsive to the detected signals andthe quantized signals for providing signals corresponding to theselected non-linear radar range scale; and memory means connected to thenon-linear range scale signal means for storing the range scale signalsand for applying said signals to a display means.
 2. An arrangement asdescribed by claim 1, wherein:the clock generator means is operative ina sequence to provide the quantized signals at a predetermined clockfrequency for a predetermined number of cycles and then switches toprovide said quantized signals at other lower predetermined clockfrequencies for other predetermined numbers of cycles for apredetermined number of different frequencies; and a plurality of radarrange bins being produced at each switching of the clock generator. 3.An arrangement as described by claim 2, wherein the clock generatormeans includes:means for counting the plurality of radar range bins. 4.An arrangement as described by claim 3, wherein the clock generatormeans includes:means connected to the counting means for monitoring saidcounting means to detect switchings of the clock generator and forproviding logic outputs upon said switchings being detected.
 5. Anarrangement as described by claim 4, wherein the clock generator meansincludes:register means connected to the monitoring means for holdingsaid logic outputs.
 6. An arrangement as described by claim 5, whereinthe clock generator means includes:programmable means connected to theregister means and programmed so as to provide different outputfrequencies in response to the logic outputs from the register means,with said programmable means providing an overflow output when the logicoutputs are all at the same logic level.
 7. An arrangement as describedby claim 3, including:the counting means providing an overflow outputwhen a predetermined number of radar range bins have been counted; andthe clock generator means including means responsive to the overflowoutput from the counting means for resetting the clock generator meansto start another sequence thereof.
 8. An arrangement as described byclaim 6, including:means for providing a clock output; means connectedto the clock output means and to the programmable means and responsiveto the clock output and the overflow output from the programmable meansfor providing first and second toggle outputs.
 9. An arrangement asdescribed by claim 6, wherein:the first toggle output is applied to thecounting means for clocking said counting means.
 10. An arrangement asdescribed by claim 8, wherein:the second toggle output is applied to themeans responsive to the overflow output from the counting means and tothe register means for clocking both of said means.